Systems and methods for purification of liquids

ABSTRACT

The present disclosure provides methods and systems for purifying liquids. In a particular implementation, the system includes a forward-osmosis unit for diluting a water source for a downstream desalination unit. A pretreatment unit may be located hydraulically upstream of the desalination unit, such as upstream or downstream of the forward-osmosis unit. In certain embodiments, the system includes an extraction unit for extracting a relatively easily extractable osmotic agent from an osmotic draw solution. The system may include one or more forward-osmosis units downstream of the desalination unit for diluting a concentrated brine stream produced by the desalination unit. In particular embodiments, a downstream forward-osmosis unit uses the concentrated brine stream as an osmotic agent, such as to extract water from seawater or brackish water. Another downstream forward-osmosis unit may use impaired water as a feed stream.

CROSS REFERENCE TO RELATED APPLICATION

This application claims the benefit of, and incorporates by reference,U.S. Provisional Patent Application Nos. 60/634,609 filed Dec. 8, 2004,and 60/634,026 filed Dec. 6, 2004.

FIELD

This disclosure pertains to liquid-treatment apparatus and methods.Particular embodiments provide such methods and apparatus as usable forproducing potable or otherwise cleaned (and hence useful) water from asource of non-potable or otherwise impaired water.

BACKGROUND

As the demand for water has grown, industry has long sought processesfor the desalination of salt water, such as seawater or brackish water.Some processes that have been used to desalinate water are distillationand membrane processes, such as reverse osmosis, nanofiltration, andelectrodialysis.

Water recovery is a major economic parameter of drinking waterproduction. However, this parameter is typically limited in existingprocesses. In addition to limited water recovery, another drawback isthat these processes are typically considered energy intensive. Membranebased systems can suffer from additional problems. For example, membranefouling and scaling in pressure-driven membrane processes (e.g., inreverse osmosis and nanofiltration) are often a major area of concern,as they can increase the cost of operating and maintaining the systems.Pretreatment of the feed water is a way of reducing fouling and scaling,but is typically expensive. An additional drawback of mostmembrane-based systems is that increased salt content of the feed streamtypically reduces the flux of product water due to the lower osmoticpotential difference between the feed solution and the permeate.

Seawater desalination has become a common practice to supply the growingdemand for water in areas having access to the sea. Shortage of potable(drinking) water in inland areas pose much more complicated challengesto water authorities, governments, and other stakeholders. Inlandregions are restricted to the use of surface water and groundwater.

SUMMARY

Embodiments of the present disclosure provide systems and methods forpurifying a liquid, such reducing its solute load. In particularimplementations, the liquid to be purified is seawater, brackish-water,impaired-water, wastewater, or other source (generally referred to assource water). In further implementations, the source water is purifiedto a potable level.

In one aspect, systems are provided for purifying a liquid, such assource water. In one embodiment, the system includes a waterpurification unit, such as a desalination unit, in combination with anupstream forward osmosis unit that dilutes a feed-water stream enteringthe desalination unit. The upstream forward-osmosis unit is locatedhydraulically upstream of the desalination unit and is configured toreceive a stream of high salinity source water. The source water passesthrough the upstream forward-osmosis unit on a receiving side of asemipermeable membrane in the upstream forward-osmosis unit. Meanwhile,a stream of liquid having a relatively low osmotic potential (e.g., aliquid having a low salinity compared to the high salinity source water)passes through the upstream forward-osmosis unit on a feed side of themembrane, which results in a net transfer of water through the membranefrom low osmolality liquid water to the source water, diluting thesource water. In a particular implementation, the resulting dilutedsource water is used as a feed for the desalination unit. By dilutingthe feed stream entering the desalination unit, the energy expenditure(per unit of product water) of the desalination unit can be reduced. Thedesalination unit produces a stream of product water and a stream ofbrine concentrate.

A further embodiment of the system includes the components of theprevious embodiment and further includes a pretreatment unit locatedupstream or downstream of the upstream forward-osmosis unit. Thepretreatment unit treats the source water before or after the sourcewater passes through the upstream forward-osmosis unit. In particularimplementations, the pretreatment unit reduces the particulate or soluteload (or both) of the source water. In certain examples, thepretreatment unit is configured to perform one or more of coagulation,media filtration, microfiltration, ultrafiltration, beach wells,ion-exchange, chemical addition, disinfection, and other membraneprocess, in any suitable order.

In another embodiment, the system includes the components in the firstdescribed embodiment as well as a downstream forward-osmosis systemsituated hydraulically downstream of the water purification unit, suchas the desalination unit. The downstream forward-osmosis system receivesthe stream of concentrate (such as concentrated brine from thedesalination unit) from the water purification unit, dilutes theconcentrate, and optionally returns the diluted concentrate to upstreamof the water purification unit, such as a feed to the water purificationunit. As a result, feed pretreatment of the stream provided by theupstream forward-osmosis unit to the water purification unit can bereduced.

The downstream forward-osmosis system can be configured as a two-stageforward-osmosis system including a first-stage forward-osmosis unit anda second-stage forward-osmosis unit connected in tandem. In a particularimplementation, each of these forward-osmosis units includes arespective semipermeable membrane. In particular examples, thefirst-stage forward-osmosis unit performs forward-osmosis of theconcentrate from the water purification unit, such as a liquid brinestream from the desalination unit, against a feed stream, such assweater, brackish water, or other source water. Because of therelatively high osmotic potential of the concentrate stream, seawater orother suitable source water having a high solute load can be used as afeed, while still providing net transfer of water across the respectivemembrane to the concentrate stream. Thus, the concentrate is diluted.

In further implementations, the second-stage forward-osmosis unitperforms forward-osmosis of the output of the first-stageforward-osmosis unit against a feed stream of source water having alower solute load than the output of the first-stage forward-osmosisunit, such as impaired water. The output of the second-stageforward-osmosis unit can be circulated back to upstream of the waterpurification unit to serve as feed to the water purification unit.

In certain aspects, the downstream forward-osmosis system can be used,after a first cycle of operation of the treatment system, to supply aportion, desirably most, of the feed water to the water purificationunit. Any required additional feed water can be supplied as make-upwater from the upstream forward-osmosis unit. Supplying at least most ofthe feed to the water purification unit from the downstreamforward-osmosis system can minimize dependence of the system on initialpretreatment and promote savings in capital equipment and operatingcosts.

According to a further embodiment, the system is similar to the previousembodiment but includes an energy-recovery system situated downstream ofthe water purification unit and upstream of the downstreamforward-osmosis system. The energy-recovery system can be, for example,a heat-exchanger if the water purification unit is a thermal-waterpurification device. Alternatively, the energy-recovery system can be apower exchanger (if the water purification unit is a pressure-drivenunit). In a particular implementation, the energy-recovery system can beany of various other energy-extracting devices that extract usableenergy from liquid passing through it. In a further implementation, theenergy-recovery system is a combination of multiple energy-extractingdevices as required or desired. In particular implementations, thesystem does not include the forward-osmosis unit upstream of thedesalination unit. In a yet further implementation, the system does notinclude the pretreatment unit.

In a particular implementation, the system includes a purification loop,such as a desalination process that can provide a higher water recoverythan reverse osmosis. In a more particular implementation, thepurification loop is a membrane distillation purification loop, such asa desalination loop, situated upstream of the downstream forward-osmosissystem. The membrane distillation purification loop can extractadditional product water from a concentrate stream produced by the waterpurification unit, such as a stream of brine produced by a desalinationunit. The membrane distillation purification loop includes a membranedistillation unit that produces a product-water stream that, whencombined with product water produced by the water purification unit, canfurther increase overall water recovery from the system and enhance theefficiency of the downstream forward-osmosis system.

In a more particular implementation, the membrane distillationpurification loop is an enhanced membrane distillation purification loopincluding an enhanced membrane distillation device exhibiting relativelyhigh flux across a membrane in the distillation device. In a particularimplementation, the enhanced membrane distillation device includes avacuum enhanced direct contact membrane distillation device. In thedevice, vacuum may be applied to the permeate side, and optionally thefeed side, of a flow cell containing the membrane in order to cause thestream to flow under vacuum or reduced pressure and enhance the flux ofliquid vapor across the membrane.

A system according to a sixth embodiment is similar to any of the firstthrough fifth embodiments where the water purification unit (of thefirst embodiment) is an osmotic desalination unit using a relativelyeasily extractable draw solution (also known as osmotic agent orreceiving solution) having a relatively high osmotic potential. In aparticular implementation, the draw solution includes ammoniumbicarbonate. In a further implementation, the draw solution includespotassium nitrate. In a particular example, product water is extractedfrom the potassium nitrate draw solution using sulfur dioxide in a drawsolution. In a further implementation, the draw solution isMagnetoferritin that can be extracted by magnetic fields.

After passing through the osmotic desalination unit, the solute may beremoved from the diluted draw solution. The method of removal may varyaccording to the nature of the draw solution. For example, when the drawsolution includes ammonium bicarbonate, the diluted draw solution may beheated to produce ammonia and carbon dioxide. The solute may be removedfrom the draw solution through various methods, including columndistillation, membrane distillation, a vacuum, a gas stream, filtration,sedimentation, precipitation, and centrifugation. For other solutes,including sulfur dioxide and potassium nitrate, separation of the drawsolution may be accomplished by taking advantage of thetemperature-dependent solubility characteristics of the draw solution,such as the solubility of a gas or a solid in the solvent of the drawsolution. For other solutes, including Magnetoferritin, magnetic fieldcan be used separate the solute from the product water. After removal ofthe solute, the product water may be used or subjected to furthertreatment, as desired. For other draw solutions, other methods ofremoving the solute may be used, including chemical, physical, orbiological methods.

Once the solute is removed from the diluted draw solutions, are-concentrating unit may be used to produce a draw solutions for re-usein the osmotic desalination unit. A heat exchanger, other suitableenergy recovery device, or heating or cooling device may be locatedupstream or downstream of the re-concentrating unit to enhance there-concentration process or the recovery of energy.

The use of an osmotic desalination unit may be advantageous compared toother desalination techniques, such as reverse osmosis, because theosmotic desalination unit may be less susceptible to membrane fouling.Furthermore, the reduced susceptibility to membrane fouling may reducethe need to pre-treat the feed stream for the osmotic desalination unit.Moreover, certain osmotic agent solutions (such as ammonium bicarbonate,sulfur dioxide, or potassium nitrate), at high but producibleconcentrations, have osmotic pressures much higher than that ofseawater—potentially resulting in a high recovery or flux of water.

Because the solute from the draw solutions may cross the forward-osmosismembrane and enter the concentrate from the osmotic desalination unit,downstream treatments may be used to recover the solute for reuse in thedraw solutions. For example, the membrane distillation loop of the fifthembodiment may be used to remove the solute, which can then be returnedto a re-concentrating unit, if desired.

The above described systems may be used for processes other than thedesalination of seawater. Other processes may include desalination ofbrackish water, concentration of foods or beverages, and concentrationor purification of chemical or pharmaceutical products.

There are additional features and advantages of the subject matterdescribed herein. They will become apparent as this specificationproceeds.

In this regard, it is to be understood that this is a brief summary ofvarying aspects of the subject matter described herein. The variousfeatures described in this section and below for various embodiments maybe used in combination or separately. A particular embodiment need notprovide all features noted above, nor solve all problems or address allissues in the background noted above.

BRIEF DESCRIPTION OF THE DRAWINGS

The description herein makes reference to the accompanying drawingswherein like reference numerals refer to like parts throughout theseveral views, and wherein:

FIG. 1 is a schematic hydraulic diagram of a water-treatment systemaccording to the first exemplary embodiment.

FIG. 2 is a schematic hydraulic diagram of a water-treatment systemaccording to the second exemplary embodiment.

FIG. 3 is a schematic diagram illustrating a vacuum enhanced directcontact membrane distillation system according to an embodiment of thepresent disclosure.

FIG. 4 is a schematic diagram illustrating a flow cell that may be usedin the system of FIG. 3.

FIG. 5 is a schematic hydraulic diagram of a water-treatment systemaccording to the third exemplary embodiment.

FIG. 6 is a diagram illustrating a flow cell having a stack of framesand membranes between two plates that may be used in disclosedforward-osmosis or membrane distillation systems.

FIG. 7 is a diagram illustrating a flow cell having a membrane envelopeinside of a pressure vessel that may be used in disclosedforward-osmosis or membrane distillation systems.

FIGS. 8A and 8B are diagrams illustrating a flow cell having one or moremembrane covered cassettes immersed in a tank that may be used indisclosed forward-osmosis or membrane distillation systems.

DETAILED DESCRIPTION

Terms

The following terms are used herein:

“Wastewater” is water that has been used in a manner or subject to acondition in which the water has acquired a load of contaminants and/orwaste products that render the water incapable of at least certaindesired practical uses without being subject to reclamation.

“Water reuse” is a beneficial use of a treated wastewater.

“Wastewater reclamation” is a treatment of a wastewater to a degree towhich the water can be reused, yielding “reclaimed” water.

“Direct reuse” is a direct use of a reclaimed wastewater, such as foragricultural and landscape irrigation, use in industry, or use in a dualwater system.

“Indirect reuse” is the mixing, dilution, or dispersion of a reclaimedwastewater into a body of “receiving” water or into a groundwater supplyprior to reuse.

“Potable water reuse” is the use of a highly treated reclaimed water toprovide or augment a supply of drinking water.

“Direct potable reuse” is the introduction of highly treated,high-quality reclaimed water directly into a drinking-water distributionsystem.

“Indirect potable reuse” is the mixing of reclaimed water with anexisting water resource (e.g., a surface resource or a groundwaterresource) before the water from the resource is delivered to adrinking-water treatment system. The mixing can occur in a river, lake,or reservoir, or by injection into an aquifer, for example.

“Seawater” (abbreviated “SW”) is saline water from the sea or from anysource of brackish water.

“Feed water” is water, such as seawater, input to a treatment processsuch as a desalination process.

“Make-up water” is pretreated and diluted seawater used to augment adesalination loop with salt lost due to diffusion from a concentrate tothe seawater or from a concentrate to the treated wastewater during aforward-osmosis process.

“Seawater pretreatment” is a treatment of seawater destined for use asmake-up water, wherein the pretreatment includes, but is not limited to,one or more of coagulation, filtration, ion-exchange, disinfection, andany other membrane process, in the stated order or any other order.

“Treated Wastewater” (abbreviated “Treated WW”) is reclaimed wastewaterthat has been subjected to a secondary or tertiary wastewater-treatmentprocess.

“Concentrated Treated Wastewater” (abbreviated “Concentrated TreatedWW”) is a treated wastewater after water has been extracted from it,such as by an forward-osmosis process; thus, concentrated treatedwastewater typically has a higher concentration of solutes and/or othernon-water waste products than treated wastewater.

“Impaired Water” is any water that does not meet potable water qualitystandards.

“Concentrate” is a by product of a water purification processes having ahigher concentration of a solute or other material than the feed water,such as a brine by-product produced by a desalination process.

“Draw solution” is a solution having a relatively high osmotic potentialthat can be used to extract water from a solution having a relativelylow osmotic potential. In certain embodiments, the draw solution may beformed by dissolving an osmotic agent in the draw solution.

“Receiving stream” is a stream that receives water by a waterpurification or extraction process. For example, in forward-osmosis, thedraw solution is a receiving stream that receives water from a feedstream of water having a lower osmotic potential than the receivingstream.

“Product Water” is potable water produced by a system as describedherein.

In addition, the terms “upstream” and “downstream” are used herein todenote, as applicable, the position of a particular component, in ahydraulic sense, relative to another component. For example, a componentlocated upstream of a second component is located so as to be contactedby a hydraulic stream (flowing in a conduit for example) before thesecond component is contacted by the hydraulic stream. Conversely, acomponent located downstream of a second component is located so as tobe contacted by a hydraulic stream after the second component iscontacted by the hydraulic stream.

Forward Osmosis

A forward-osmosis process is termed “osmosis” or “direct osmosis.”Forward-osmosis typically uses a semipermeable membrane having apermeate side and a feed side. The feed (active) side contacts the water(feed water) to be treated. The permeate (support) side contacts ahypertonic solution, referred to as an osmotic agent or a draw solutionor receiving stream, that serves to draw (by osmosis) water moleculesand certain solutes and other compounds from the feed water through themembrane into the draw solution. The draw solution is circulated on thepermeate side of the membrane as the feed water is passed by the feedside of the membrane. Unlike reverse osmosis, which uses a pressuredifferential across the membrane to induce mass-transfer across themembrane from the feed side to the permeate side, forward-osmosis usesan osmotic-pressure difference as the driving force for mass transferacross the membrane. As long as the osmotic potential of water on thepermeate side (draw solution side) of the membrane is higher than theosmotic potential of water on the feed side, water will diffuse from thefeed side through the membrane and thereby dilute the draw solution. Tomaintain its effectiveness in the face of this dilution, the drawsolution must typically be re-concentrated, or otherwise replenished,during use. This re-concentration typically consumes most of the energythat conventionally must be provided to conduct a forward-osmosisprocess.

Because the semipermeable membranes used in forward-osmosis aretypically similar to the membranes used in reverse osmosis, mostcontaminants are rejected by the membrane and only water and some smallmolecules diffuse through the membrane to the draw solution side. Acontaminant that is “rejected” is prevented by the membrane from passingthrough the membrane. Selecting an appropriate membrane usually involvesselecting a membrane that exhibits high rejection of salts as well asvarious organic and/or inorganic compounds while still allowing a highflux of water through the membrane at a low driving force.

Other advantages of the forward-osmosis process can include relativelylow propensity to membrane fouling, low energy consumption, simplicity,and reliability. Because operating pressures in the forward-osmosisprocess typically are very low (up to a few bars, reflective of the flowresistance exhibited by the housing containing the membranes), theequipment used for performing forward-osmosis can be very simple. Also,use of lower pressure may alleviate potential problems with membranesupport in the housing and reduce pressure-mediated fouling of themembrane.

In one application, the disclosed systems and methods can be used totreat raw wastewater to make it potable. The disclosed systems andmethods can also be used in the treatment of landfill leachates, foods,and beverages. In particular implementations, more than 97% of the totalnitrogen and more than 99.5% of the phosphorus in a feed solution can berejected by disclosed methods and systems.

Forward-Osmosis-Assisted Desalination

With a suitable forward-osmosis semipermeable membrane, a relativelyhigh flux of fresh water, or water from impaired water, through themembrane into the draw solution (e.g., seawater, concentrated seawater,or other suitable hypertonic solution) can be realized. For example, adraw solution having a solute concentration close to that of seawatercan produce flux of at least 10 L/(m²·hr) of clean water through thesuitable forward-osmosis membrane into the draw solution. Thus, usingforward-osmosis, seawater can be diluted with highly treated wastewaterprior to the seawater being subject to desalination, thereby reducingthe salinity of the seawater and correspondingly reducing the energyrequired to desalinate it. The concentrated brine produced may be usedas a draw solution in downstream purification processes.

First Exemplary Embodiment

A first exemplary embodiment of a water-treatment process includes adesalination process and one or more forward-osmosis pretreatment stagesto reduce feed-water salinity and to reduce or eliminate conventionalpretreatment of the feed water. In the process a desalination step isperformed in which the feed water is diluted with fresh water. Thefresh-water diluent is supplied by forward-osmosis of treatedwastewater, run-off water, or any impaired water, for example. Althoughgenerally described in these exemplary systems for use in desalinatingsalt water, the methods and systems described in the exemplaryembodiments may be applied to other source liquids.

An exemplary apparatus 10 for performing the process is shown in FIG. 1and includes the following components: a desalination unit 12, aseawater-pretreatment unit 14, an upstream forward-osmosis unit 16comprising a forward-osmosis membrane 18, a pump 20, a seawater-feedstream 22, a wastewater (reclaimed or impaired) feed stream 24, anenergy-recovery system 26, and a dual-stage forward-osmosis system 27arranged in a loop.

The seawater-pretreatment unit 14 and upstream forward-osmosis unit 16collectively provide a water stream that may be used to provide make-upwater or start-up water to the desalination unit 12. The desalinationunit 12 can be, for example, a reverse osmosis, nanofiltration,electrodialysis, forward-osmosis, ammonium bicarbonate forward-osmosis(“ABFO” or “FO desalination”), distillation, or any other suitabledevice.

The energy-recovery system 26 can include a heat-exchanger, such ascondensers, shell and tube heat exchangers, plate heat exchangers,circulators, radiators, and boilers, which may be parallel flow, crossflow, or counter flow heat exchangers (if the desalination unit 12 is athermal-desalination device), a power exchanger (if the desalinationunit 12 is a pressure-driven desalination device), or other suitabledevice that extracts usable energy from liquid entering it. Theenergy-recovery system 26 can be a combination of these exemplarydevices as required or desired.

In the embodiment shown in FIG. 1, the dual-stage forward-osmosis system27 includes a first-stage forward-osmosis unit 28 including a firstforward-osmosis membrane 30, and a second-stage forward-osmosis unit 32including a second forward-osmosis membrane 34. The first-stageforward-osmosis unit 28 and the second-stage forward-osmosis unit 32 arearranged hydraulically in tandem in a hydraulic loop.

Seawater (or other make-up water, termed generally “seawater” here) 36is drawn from an appropriate source and passes through the pretreatmentunit 14. The pretreatment unit 14 pretreats the seawater, as required,such as subjecting it to one or more processes such as coagulation,media filtration, microfiltration, ultrafiltration, beach wells,ion-exchange, chemical addition, disinfection, and other membraneprocess, in any suitable order. The effluent make-up water 38 from thepretreatment unit 14 enters the upstream forward-osmosis unit 16.

As the make-up water 38 passes through the upstream forward-osmosis unit16 on the permeate side of the membrane 18, treated wastewater 40, orimpaired water, is circulated through the upstream forward-osmosis unit16 on the feed side of the membrane 18. As a result, the make-up water38 is diluted by transfer of water (as indicated by the “W” arrows inFIG. 1) from the feed side through the membrane 18. Thus, the treatedwastewater 24 is concentrated to produce a concentrate stream 42, andthe make-up water 38 is diluted. The diluted water stream 44 exiting theupstream forward-osmosis unit 16 is suitably pressurized by the pump 20as required by the desalination unit 12. The resulting pressurized water46 enters the desalination unit 12, which removes particulates, if any,and solutes, such as salt solutes, from the water 46 sufficiently toproduce the desired product water 48 (such as potable water). Theproduct water 48 may be subjected to further purification steps. Theremoved particulates, if any, and solutes, entrained in a concentratestream 50, pass through the energy-recovery system 26 configuredappropriately for the particular type of desalination unit 12, asdiscussed above.

The de-energized water stream 51 (now at relatively low pressure) passesthrough the dual-stage forward-osmosis system 27, namely first throughthe first-stage forward-osmosis unit 28 and then through thesecond-stage forward-osmosis unit 32. As the de-energized concentrate 51passes through the first-stage forward-osmosis unit 28 on the permeateside of the membrane 30, seawater 52 (or other suitable impaired water)is circulated through the first-stage forward-osmosis unit 28 on thefeed side of the membrane 30. As a result, the concentrate stream 51 isdiluted by transfer of water (as indicated by the “W” arrows in FIG. 1)from the feed side of the membrane 30. The concentrated brine 54 fromthe forward-osmosis unit 28 may be discharged from the first-stageforward-osmosis unit 28.

As the diluted concentrate 56 from the first-stage forward-osmosis unit28 passes through the second-stage forward-osmosis unit 32 on thepermeate side of the membrane 34, treated wastewater 58 (or otherimpaired water having a suitably low salinity) is circulated through thesecond-stage forward-osmosis unit 32 on the feed side of the membrane34. As a result, the diluted concentrate 56 is further diluted bytransfer of water (as indicated by the “W” arrows in FIG. 1) from thefeed of the membrane 34, thereby concentrating the wastewater 58, orimpaired water, in a concentrate stream 60 that is discharged from thesecond-stage forward-osmosis unit 32. The brine 62 from the second-stageforward-osmosis unit 32, now further diluted, is routed to upstream ofthe pump 20, thereby completing the loop from downstream of thedesalination unit 12 to upstream of it.

After an initial priming of the system 10, in which all the feeds to thedesalination unit 12 are passed through the upstream forward-osmosisunit 16, the system 10 runs in a manner by which at least most of thefeed water to the desalination unit 12 is supplied by the diluted brine62 from the second-stage forward-osmosis unit 32. Any required make-upwater can be provided by the upstream forward-osmosis unit 16. Supplyingat least most of the feed water to the desalination unit 12 from thetwo-stage forward-osmosis system 27 minimizes dependence of the system10 on the pretreatment unit 14, thus promoting savings in capitalequipment, maintenance, and operating costs.

As an alternative to the hydraulic circuit shown in FIG. 1, thepretreatment unit 14 can be located downstream of the upstreamforward-osmosis unit 16, in which event the pretreatment unit 14 stillwill be located upstream of the pump 20 and upstream of the connectionof stream 62 with stream 44. In other words, the make-up water 36 can bepretreated either before or after (but more desirably before) theosmosis step performed by the upstream forward-osmosis unit 16.

The seawater 36 is used as make-up water for replenishing salt lostduring desalination by the desalination unit 12, at least during systemstart-up. The seawater 36 desirably is diluted by the upstreamforward-osmosis unit 16 for feeding the desalination unit 12, at leastduring system start-up. After desalination, as noted above, theenergy-recovery system 26 recovers energy from the pressurizedconcentrate or heated concentrate 50 exiting the desalination unit 12.After recovery of energy from the concentrate 50, the resultingde-energized concentrate 51 passes through the two-stage forward-osmosissystem 27, as discussed above, in which the concentrate is diluted bywater supplied from seawater, impaired water, wastewater, run-off water,or any other impaired water by forward-osmosis.

Passing the concentrate 50 through the two-stage forward-osmosis unit 27dilutes the concentrate 50 for use as feed 46 to the desalination unit12. Since the salinity and load of solutes and other contaminants in thefeed 46 may be reduced (compared to ordinary seawater) by the two-stageforward-osmosis system 27, the desalination unit 12 can be operated at areduced pressure and/or temperature than it otherwise would have to befor producing the desired flux or volume of product water 48. Thereduced pressure and/or temperature can yield reduced rates of membraneclogging and fouling in the desalination unit 12.

Because forward-osmosis membranes and processes generally exhibit a lowdegree of fouling, forward-osmosis can be advantageously used in thisembodiment for pretreating reclaimed water or impaired water for use inmost desalination processes. This can eliminate other, more expensive,pretreatment steps as well as protect the desalination process.

The concentrate 50 expelled from the desalination unit 12 is mostlyrecycled in this embodiment, and only a small amount of salt istypically added (in the dual-stage forward-osmosis system 27 and fromthe upstream forward-osmosis unit 16 as required) to compensate forlosses through the forward-osmosis membranes and the desalination unit12. This is an advantage because, as a result, the desalination unit 12is not exposed to substantial amounts of new foulants. Moreover, asolution exhibiting a very low scaling tendency can be specificallyselected as an osmotic agent in the forward-osmosis units 28, 32, whichmay reduce the need for use of scale inhibitors.

In the two-stage forward-osmosis system 27, the forward-osmosisperformed with seawater dilutes the concentrate stream to below thenormal level of seawater salinity. This produces feed water having alower osmotic pressure for the desalination unit 12. Similarly, in theupstream forward-osmosis unit 16, the forward-osmosis performed withtreated seawater dilutes the seawater to below the normal level ofseawater salinity, providing feed water having a lower osmotic pressurethan seawater for the desalination unit 12. As a result, the energyrequired by the desalination unit 12 for performing desalination can belowered or the overall water-recovery or flux of the system 10 enhanced.

Although in this embodiment the forward-osmosis system 27 is depictedand described as a “two-stage” forward-osmosis system, it will beunderstood that this forward-osmosis system alternatively can includeonly one forward-osmosis unit or can include more than twoforward-osmosis units. In addition, even though the forward-osmosissystem 27 is shown and described with the forward-osmosis units beingconnected in tandem (in series), it will be understood that otherinterconnection schemes (including parallel connection schemes and/orcombinations of parallel and series) can be used.

Another potential advantage of this embodiment is that advancedpretreatment (by the pretreatment unit 14) is performed on only theminimal volume of seawater 36 that is required for making up for saltlosses in the system 10. Yet potential advantage of this embodiment isthat water streams that would be otherwise typically be treated aswaste, such as concentrated brine from the desalination unit 12, can beused to create more product water or lower the capital, maintenance, orenergy costs of the system.

It may be desirable to post-treat the product water 48. The particularnature of the post-treatment may depend on the use of the product water48. In one implementation, the product water 48 can be subjected to oneor more of pH adjustment (such as by suitable titration), chlorination,ozonation, UV irradiation, ion exchange, activated-charcoal adsorption,or the like.

It will be understood that this embodiment can be used for purposesother than desalination of seawater or of impaired water. The disclosedembodiment can be used for treating raw wastewater to drinking-waterlevel. The disclosed embodiment may also be used in the treatment oflandfill leachates. The disclosed embodiment can also be used in thefood industry or in feed solutions as used in the chemical industry,pharmaceutical industry, or biotechnological industry. In particularimplementations, more than 97% of the total nitrogen and more than 99.5%of the phosphorus in the feed solution are rejected by the disclosedsystems.

Second Exemplary Embodiment

A system 80, which is similar to the system of FIG. 1 in many respects,is depicted in FIG. 2. Components of the system 80 shown in FIG. 2 thatare the same as respective components of the system 10 shown in FIG. 1have the same respective reference designators and are not describedfurther except as noted below.

The system 80 of FIG. 2 includes a dual-stage forward-osmosis device 27comprising a first-stage forward-osmosis unit 28 and a second-stageforward-osmosis unit 32, as in the first exemplary embodiment. FIG. 2shows the first-stage forward-osmosis unit 28 being supplied withseawater 52 (as a feed water) by a respective pump 82, and thesecond-stage forward-osmosis unit 32 being supplied with impaired water84 (as a feed water) by a respective pump 86. Similarly, theforward-osmosis unit 16 upstream of the desalination unit 12 is suppliedwith impaired water 40 (as feed water) by a respective pump 88.

In the dual-stage forward-osmosis device 27, the concentrated drawsolution 50 produced by the desalination unit 12 contacts the receivingside of the forward-osmosis membrane 30 and seawater (or other suitablewater) contacts the feed side of the forward-osmosis membrane 30 in thefirst-stage forward-osmosis unit 28. Water passing through the membrane30 from the feed side to the receiving side dilutes the draw solution.The diluted draw solution 56 exiting the first-stage forward-osmosisunit 28 then enters the second-stage forward-osmosis unit 32, in whichthe diluted draw solution 56 contacts the permeate side of theforward-osmosis membrane 34 and impaired water 84, or another suitablewater source, contacts the feed side of the forward-osmosis membrane 34.Both forward-osmosis stages 28, 32 are used to induce seawater (or otherfeed water) dilution of a draw solution to be used again as feed water62 to the desalination unit 12.

The embodiment 80 shown in FIG. 2 also includes a membrane distillationdesalination device 90 that is used to extract additional product waterfrom the concentrated draw solution 50 produced by the desalination unit12. The membrane distillation desalination device 90 produces aproduct-water stream 92 and returns spent concentrated draw solution toconcentrate 50 to serve as the draw solution in the first-stageforward-osmosis unit 28. The membrane distillation desalination device90 is typically relatively insensitive to the salt concentration of thefeed solution. Thus, the membrane distillation desalination device 90can further increase overall recovery or flux of product water 48, 92from the system 80 and enhance the efficiency of the dual-stageforward-osmosis device 27.

In at least one embodiment, the membrane distillation desalinationdevice 90 is an enhanced membrane distillation desalination device thatis able to produce relatively high flux across a membrane (not shown).In a particular implementation, the enhanced membrane distillationdesalination device is a direct-contact membrane-distillation device. Ina more particular implementation, the enhanced membrane distillationdesalination device 90 uses an enhanced membrane distillation methodwhereby vacuum is applied to a permeate side, and optionally a feedside, of a flow cell (not shown) containing the membrane to cause thestream to flow under vacuum or reduced pressure.

The system 80 can be configured to be more energy efficient. Forexample, if the desalination unit 12 is pressure-driven (such asnanofiltration or reverse osmosis), the energy-recovery system 26 in thesystem 80 may include a “pressure-retarded power exchanger” 94.Alternatively, if the desalination unit 12 is thermally driven, theenergy-recovery system 26 desirably includes a heat-exchanger (HX) forrecovering heat from the concentrate. Suitable heat exchangers includecondensers, shell and tube heat exchangers, plate heat exchangers,circulators, radiators, and boilers and may be parallel flow, crossflow, or counter flow heat exchangers.

FIG. 3 illustrates a suitable membrane distillation device that may beused in the Second Exemplary Embodiment. A vacuum enhanced directcontact membrane distillation system 100 is shown having a feed source110 containing a feed solution 114, such as an impaired liquid. In aparticular example, one or more solutes 116 are dissolved in the feedsolution 114. When the system 100 is used for desalination, the impairedliquid may be seawater or brackish water. Although the feed source 110is shown as an isolated tank, any suitable feed source 110 can be used,such as a feed stream from another system or from an intake incommunication with a feed source, such as a body of water, for example,an ocean.

The feed solution 114 optionally may be pumped through a pump 120, whichmay be configured to control the flow rate or to apply positive ornegative pressure, as desired. The pump 120 may also be used to causethe feed stream to flow on a membrane 136. One suitable pump 120 forlaboratory scale application is the model 1605A pump, available fromProcon Pumps of Murfreesboro, Tenn. The flow rate may be furthercontrolled using a valve 124.

The feed solution 114 is transported from the feed source 110 to a flowcell 128. The flow cell 128 has a feed side 130 and a permeate side 132.The membrane 136 is disposed between the feed side 130 and the permeateside 132. The flow cell 128 may be constructed from any suitablematerial, including polymers. In particular example, the flow cell 128is formed from acrylic.

The membrane 136 can be a hydrophobic membrane. In particular examples,microporous hydrophobic membranes 136 are used. The microporoushydrophobic membrane 136 may have pores of any suitable size, such aspore sizes of about 0.03 to about 0.5 microns, such as pore sizes ofabout 0.2 to about 0.45 microns.

The membrane 136 may be made from one or more suitable hydrophobicmaterials, such as hydrophobic polymers. In particular implementations,highly hydrophobic membranes are used. In further implementations, themembranes are relatively thinner and more porous. Exemplary membranes136 may be constructed from Teflon or polypropylene (PP).

The membrane 136 may have one layer or multiple layers. For example, themembrane 136 may have one or more active layers and one or more supportlayers. In a particular embodiment, the membrane 136 has a thinpolytetrafluoroethylene (PTFE) active layer and a polypropylene (PP)support sublayer. For membranes 136 having an active layer and a supportlayer, the active layer typically is positioned facing the feed side 130of the flow cell 128. Suitable hydrophobic microporous membranes may beobtained from Osmonics Corp. of Minnetonka, Minn. Suitable membranes andtheir properties are summarized in Table 1 below.

Nominal Active layer pore Porosity Thickness thickness Membrane Materialsize (μm) (%) (μm) (μm) PS22 PP 0.22 70 150 150 TS22 PTFE 0.22 70 1755-10 TS45 PTFE 0.45 70 175 5-10 TS1.0 PTFE 1.0 70 175 5-10

Although the membrane 136 is shown as flat, other shapes andconfigurations may be used for the membrane 136. For example, a flatmembrane 136 may be encased, or otherwise supported, in order to helpmake the membrane 136 more robust.

In at least one embodiment, the flow cell 128 is constructed such thatthe membrane 136 is held in place in the flow cell 128 by friction orpressure, such as being sandwiched between the feed side 130 and thepermeate side 132 of the flow cell 128. However, other means of securingthe membrane 136 could be used, such as various fastening or adhesivemeans, such as tape, glue, clamps, clasps, clips, pins, or screws. Fluidpressure may be used to help keep the membrane 136 from collapsing whenvacuum is applied to the feed side 130 or the permeate side 132 of theflow cell 128. The flow cell 128 is typically constructed such that themembrane 136 does not collapse during operation.

Because temperature and pressure can affect the flux of permeate passingfrom the feed side 130 to the permeate side 132 of the flow cell 128,thermocouples 140, 142 and pressure gauges 144, 146 may be included onthe output and input sides, respectively, of the feed cycle 150. A flowmeter 152 is located on the output end of the feed side 130 of the flowcell 128.

Turning now to the permeate cycle 160, permeate passing through themembrane 136 condenses into a permeate stream 168 and is conducted to apermeate reservoir 164. The permeate reservoir 164 is shown as adiscrete tank, but the permeate reservoir 164 could be other types ofreservoirs. The permeate reservoir 164 could also be a transport devicefor carrying the permeate solution 168 to another system or location.The permeate solution 168 is typically a solution containing less solute(is more dilute) than the feed solution 114. When the permeate 168 iswater, in particular implementations, the permeate stream is distilledwater, de-ionized water, potable water, runoff water, or other waterhaving a relatively low amount of total dissolved solids.

In particular examples, a relatively low amount of total dissolvedsolids is less than about 1,000 mg/l of total dissolved solids, such asless than about 500 mg/l. In a more particular example, the amount oftotal dissolved solids in the permeate stream is an amount that, whenmixed with water crossing the membrane 136, produces a product waterhaving a concentration of total dissolved solids of less than about 500mg/l. In a particular example, the concentration of total dissolvedsolids in the permeate 168 is between about 200 mg/1 and about 500 mg/l.

The permeate loop 160 includes a vacuum pump 170 for placing thepermeate side 132 of the flow cell 128 under vacuum, which may be lowerthan ambient pressure or lower than the pressure of the feed loop 150.The pump 170 may also cause the permeate solution 168 to flow over themembrane 136. The vacuum pump 170 may be of any suitable type to producethe range of pressures desired, typically between 0.1 and 1.0atmospheres, such as between about 0.5 about 1.0 atmospheres. Onesuitable pump 170 for small laboratory application is the model 1605Apump, available from Procon Pumps of Murfreesboro, Tenn.

As with the feed loop 150, the permeate loop 160 may be provided withthermocouples 174, 176 and pressure gauges 178, 180 at the input andexits ends, respectively, of the flow cell 128 in order to monitor orcontrol the pressure or temperature of the permeate loop 160. A bypassvalve 184 is provided to assist in controlling the pressure of thepermeate loop 160. A front valve 186 is located proximate the input endof the permeate side 132 of the flow cell 128 to provide further controlof the flow rate of the permeate solution 168 and the pressure of thepermeate cycle 160. A flow meter 188 is provided at the output of thepermeate side 132 of the flow cell 128.

According to a particular method of the present invention, the system100 is operated while applying a vacuum to the permeate cycle 160. In aparticular implementation, the vacuum may be any pressure less thanatmosphere pressure. In a further implementation, the vacuum may be anypressure less than the pressure of the feed loop 150. The temperature ofthe feed solution 114 is typically maintained higher than thetemperature of the permeate solution 168, such as at least about 5degrees higher, such as at least 10 degrees higher.

For water based feed solutions 114 and permeate solutions 168, greatertemperature differentials between the feed solution 114 and the permeatesolution 168 generally result in higher flux across the membrane 136.Increased feed solution 114 temperature generally increases the fluxacross the membrane 136 due the increased vapor pressure of the feedsolution. Increased vacuum (lower pressure) on the permeate cycle 160also typically increases the flux across the membrane 136.

In operation, relatively warmer feed solution 114 enters the feed side130 of the flow cell 128. Vapor from the feed solution 114 enters thepores of the membrane 136 and flows to the permeate side 132 of the flowcell 128. The permeate vapor condenses into the permeate solution 168and is carried out of the flow cell 128 for recovery. The system 100 istypically run as a continuous process.

FIG. 4 is a detailed view of a system 200 including a flow cell 210 thatmay be used in the vacuum enhanced direct contact membrane distillationsystem 100 of FIG. 3. The flow cell 210 has a feed compartment 214 thatreceives a feed solution 218 from a feed tank 220. A pump 224 is used tocontrol the flow of the feed solution 218.

The feed solution 218 enters the feed compartment 214 through an inletport 226. Narrow channels (not shown in FIG. 2) are used to transportthe feed solution 218 to flow channels 228 formed in the feedcompartment 214. In at least one embodiment, the flow channels 228 havea width of between about 1 and about 5 millimeters. In particularimplementations, the flow cell 210 is designed to have the feed solution218 flowing with a high Reynolds number. In more particularimplementations, the flow cell 210 is designed such that high turbulence(reflected by the high Reynolds number) may be achieved at relativelylow pressure (for example, 30-40 psi). In addition, the feed solution218 may be used to provide support for a membrane 232 so that themembrane 232 does not collapse during operation of the system 200. Afterpassing through the flow channels 228, the feed solution 218 passes outof the flow cell 210 through an outlet port 234.

The flow cell 210 has a permeate (product) compartment 244 which abutsthe feed compartment 214. The membrane 232 is positioned between thepermeate compartment 244 and the feed compartment 214. In certainembodiments, the membrane 232 is a flat sheet membrane. In particularembodiments, the membrane 232 is supported by fluid on the both sides ofthe flow cell 210. However, other support means or affixing means couldbe used, if desired, to secure the membrane 232 in position, such asadhesive or fastener means, including tape, glue, screws, clips, clasps,clamps, or pins.

A permeate solution 248 is stored in a permeate tank 250. A pump 254 anda valve 260 can be used to control the flow velocity of the permeatesolution 248 and the pressure of the permeate cycle. The permeatecompartment 244 is constructed similarly to the feed compartment 214,including the arrangement and construction of flow channels (not shown).The permeate solution 248 enters the permeate compartment though aninlet 268 and exits the permeate compartment though an outlet 270.

Third Exemplary Embodiment

A system 300 is illustrated in FIG. 5. Components of the system 300shown in FIG. 5 that are the same as respective components of the system10 shown in FIG. 1, or the system 80 shown in FIG. 2 have the samerespective reference designators and are not described further except asnoted below. The system of FIG. 5 will be described in conjunction withcomponents of the system of FIG. 2, but could be used in other systems,including the system of FIG. 1.

The system 300 of FIG. 5 includes a dual-stage forward-osmosis device 27having a first-stage forward-osmosis unit 28 and a second-stageforward-osmosis unit 32, as in the first and second embodiments. As inthe second embodiment, the first stage forward-osmosis unit 28 includesa membrane distillation device 90 to extract additional product waterfrom the concentrate 50 produced by the desalination unit 12. Themembrane distillation device 90 also produces more concentrated brinefor use as the draw solution in the first-stage downstreamforward-osmosis unit 28, thus increasing the efficiency of thefirst-stage forward-osmosis unit 28.

In the embodiment of FIG. 5, the desalination unit 12 is an osmoticdesalination unit where a concentrated, easily extractable, drawsolution is used to draw water from the feed 46. The osmoticdesalination unit 12 is part of an osmotic desalination system 310. Anysuitable draw solution, osmotic desalination unit 12, and osmoticdesalination system 310 can be used.

The draw solution used in the osmotic desalination system 310, inparticular implementations, is a relatively easily extractable osmoticagent that requires relatively low energy, or a relatively simplemechanism, for separation and is relatively easily recovered from thediluted osmotic agent solution 316. For example, the osmotic agent maybe precipitated from the solution or converted to a gas and driven outof the diluted draw solution 316 or it can be Magnetoferritin particlesextracted by magnetic field. In certain implementations, the drawsolution has a solubility that is substantially temperature dependent,such as potassium nitrate, sulfur dioxide, or ammonium bicarbonate. Infurther embodiments, the osmotic agent may separated with differentforces such as a magnetic field or an electromagnetic field. In furtherimplementations, the draw solution is in equilibrium with one or moreother species. Removal or concentration of the draw solution can beaccomplished by driving the equilibrium to favor the desired species.Accordingly, the draw solution can be removed from the diluted osmoticagent solution 316 to produce product water 318. The draw solution maybe recycled for use in the osmotic desalination unit 12.

As shown in FIG. 5, feed water 46 enters the osmotic desalination unit12 and contacts one side of a membrane (not shown). The other side ofthe membrane contacts an osmotic agent solution 322. In a particularembodiment, the draw solution 322 is formed by dissolving ammonia andcarbon dioxide gas. When these gasses are dissolved, a solution willform containing ammonium carbonate ((NH₄)₂CO₃), ammonium bicarbonate(NH₄HCO₃), and ammonium carbamate (NH₂COONH₄). Alternatively, thesespecies could be directly added to the draw solution 322, such as insolid form or in solution.

Le Chetalier's Principle may be used to drive the equilibrium to favor aparticular species, such as by buffering the solution with an excess ofammonia gas. Doing so will drive the equilibrium to favor the moresoluble ammonium carbamate species over the less soluble ammoniumbicarbonate. In one example, the ratio of ammonia to carbon dioxide isabout 1.75 to 2.0. In particular implementations, the totalconcentration of solute in the draw solution 322 is greater than about 2molal, such as greater than about 6 molal, such as between about 6 molaland about 12 molal. If desired, the concentration of the draw solution322 can be further increased, such as by raising its temperature, inorder to allow more solute to be solubilized. For example, thetemperature can be raised to about 50 to about 55° C., thus allowing upto about an 18 molal solution to be formed. When ammonium carbamate isused as a draw solution, the temperature of the solution is typicallykept below about 58° C., above which it may decompose.

Because of the higher osmotic potential of the draw solution 322, waterwill diffuse through the membrane from the feed water 46 and into thedraw solution 322, thus diluting it. A desired rate of flux can beachieved by proper selection of the concentration of the draw solution322. The flux rate can also be controlled by the nature and size of themembrane used. For example, a smaller membrane can still giveappreciable flux if the draw solution 322 is sufficiently concentrated.Any suitable semi-permeable membrane may be used, such as those madefrom organic materials such as cellulose nitrate, polysulfone, celluloseacetate, polyvinylidene fluoride, polyamide, and acrylonitrileco-polymers. Mineral or ceramic membranes may also be used, such asthose made from zirconate or titinate. The membrane is typically chosento withstand the chemical and physical environment in which it will beused, such as the pH, temperature, and pressure used. In particularexamples, the membrane is a model AG or CE reverse osmosis membraneavailable from GE Osmonics of Trevose, Pa. Other suitable membranes,including forward osmosis membranes, are available from Hydrationtechnologies of Albany, Oreg.

After exiting the osmotic desalination unit 12, the concentrate(concentrated brine) 328 enters a heat exchanger 332 which can recoverenergy from the concentrated brine 328. Suitable heat exchangers includecondensers, shell and tube heat exchangers, plate heat exchangers,circulators, radiators, and boilers and may be parallel flow, crossflow, or counter flow heat exchangers. The concentrated brine 328 thenproceeds to the dual-stage forward-osmosis system 27, as was discussedin the Second Exemplary Embodiment.

After leaving the osmotic desalination unit 12, carbon dioxide mayoptionally be added to the diluted osmotic agent solution 316 in orderto increase the concentration of the relatively insoluble ammoniumbicarbonate. Precipitated ammonium bicarbonate can optionally berecovered from the diluted draw solution 316 and recycled. Means forrecovering solid materials are well known in the art and includesedimentation tanks, screen filtration, column filtration,hydrocyclones, or nucleation point, such as a precipitation mass. Thetemperature of the diluted draw solution 316 may be optionally cooled,such as to about 18 to 25° C., such as to about 20 to 25° C. Cooling thediluted draw solution 316 may increase the amount of osmotic agentprecipitated from the diluted draw solution 316.

The diluted draw solution 316, typically with a portion of the soluteremoved, enters a heat exchanger 336 where the diluted draw solution 316is heated. Upon heating, the ammonium bicarbonate in the diluted drawsolution 316 will decompose into carbon dioxide and ammonia. In certainimplementations, the diluted draw solution 316 is heated to about 30 toabout 100° C., such as from about 30 to about 60° C., such as to about60°. The diluted draw solution 316 passes from the heat exchanger 336 toan ammonia- and carbon-dioxide-recovery device 340. The recovery device340 may be any suitable device for extracting the carbon dioxide andammonia from the diluted draw solution 316. In particular examples, therecovery device 340 is a column distillation unit or a membranedistillation unit. A vacuum or stream of air may be included in therecovery device 340 to aid in removing the gasses driven out of thediluted draw solution 316. The recovered ammonia and carbon dioxide gascan then be used in the processes described above for altering theequilibrium of the draw solution.

From the recovery device 340, product water 318 is produced and thecarbon dioxide and ammonia are used to re-concentrate the draw solution322 in a concentration unit 348. The re-concentrated draw solution 322then enters a heat exchanger 352 to enhance the re-concentration processand to recover energy from the solution.

Because of the high concentration of ammonium bicarbonate used in thedraw solution 322, some ammonia may diffuse through the membrane of theosmotic desalination unit 12. When used in conjunction with the processdescribed herein, most of the ammonia lost to the concentrate 51 staysin the system 300 and can be later harvested in one or more processes ofthe system 300. For example, in at least one embodiment, the ammonia inthe concentrate 51 may be separated from the concentrate 51 during themembrane distillation process 90. The recovered ammonia 350 can bereturned to the recovery device 340, if desired. In contrast to certainprevious desalination techniques where brine (concentrate from thedesalination) is directly discharged back to the sea/ocean, in theproposed configuration most of the ammonia nitrogen is not released tothe environment.

As previously mentioned, the present disclosure is not limited ammoniumbicarbonate draw solution 322. Any extractable solute having a highosmotic potential can be used as the draw solution and any suitableextraction technique can be used. For example, U.S. Pat. No. 6,391,205,which is expressly incorporated by reference in its entirety, discusesdesalination of water using a first draw solution, such as KNO₃, Na₃PO₄,or sucrose, having a solubility that increases with increasingtemperature. After extracting water from the feed stream, the diluteddraw solution is transferred to a heat exchanger to cool the solution,and the draw solution is removed. The diluted draw solution is thensubjected to another forward osmosis step using a draw solution having asolubility that decreases with decreasing temperature, such as sulfurdioxide gas. After this extraction step, the sulfur dioxide can beremoved by heating the product water. Residual sulfur dioxide can beremoved from the product water, such as with an acid removal system,such as a lime bed.

The above described exemplary embodiments may be implemented in anysuitable manner, which may depend on the particular application,including the scale of the application. The various components, such asheat exchangers and purification units, may be made of suitablynon-reactive materials such as plastic, PVC, stainless steel,fiberglass, and PVC. Liquid sources or other vessels may be cylindricaltanks, water towers, contoured tanks, or fitted tanks.

Flow Cell Configurations

The present disclosure provides a number of flow cell configurationsthat may be used in the forward-osmosis or membrane distillation unitsof the disclosed systems. The components of the flow cells are generallysimilar whether they are used for membrane distillation or forwardosmosis. The membrane used in the flow cell is selected for theparticular flow cell application. For example, membranes used inmembrane distillation flow cells are typically hydrophobic. Hydrophilicmembranes are typically used in forward-osmosis flow cells. Inparticular implementations, the membranes used in the disclosed flowcells are hollow fiber or tubular membranes.

With reference to FIG. 6, a number of gaskets 410 and membranes 420 maybe included between two plates 424, 426 in order to form a stack 430. Inoperation, feed solution enters the flow cell 400 through an inlet port434 formed in the plate 424. The feed solution will flow through a flowduct 438 of a gasket 442 and then into flow channels 446 of the gasket442.

The flow channels 446 are typically constructed such that highturbulence (such as indicated by a relatively large Reynolds number,such as a Reynolds number of at least about 2300, for example, aReynolds number of at least about 5000) can be achieved. Particularlywhen used for membrane distillation, in at least certainimplementations, the flow channels 446 are contructed such that hightubrulence is achieved at relatively low pressure (for example, 30-40psi). The flow channels 446 also are preferably constructed to providesupport for the membranes 420. In at least one embodiment, the flowchannels 446 have of width of between about 1 mm and about 5 mm. Ofcourse, the construction of the flow cell 400, including the dimensionsand orientation of the flow channels 446, the turbulence achieved, andthe operational pressure can be varied according to the needs of aparticular application.

After flowing through the flow channels 446 of the gasket 442, the feedsolution enters a flow duct 450 in the gasket 442, passes through anopening (not shown) in a membrane 460, and enters a flow duct (notshown, at least substantially congruent with the flow duct 450) in agasket 458. The solution will then be conducted, without passing throughthe flow channels (not shown, at least similar to flow channels 446) inthe gasket 458, through an opening in a membrane 461 and into a flowduct in a gasket 462. The feed solution will flow through the flowchannels (not shown) in the gasket 462.

A draw solution enters the flow cell 400 through an inlet 464 in theplate 424 and into a flow duct 466. From the flow duct 466, the permeatesolution is directed through a corresponding opening (not shown) in themembrane 460 and into a flow duct (not shown, at least substantiallycongruent with the flow duct 466) of the gasket 462. The permeatesolution will be conducted into flow channels (not shown, at leastsimilar to flow channels 446).

The feed and draw solutions continue to flow in this way through all ofthe membranes 420 and the gaskets 410 in the stack 430. The drawsolution and feed solutions thus flow through alternate gaskets 410. Themembranes 420 are oriented such that if the membrane 410 has an activesurface, the active surface is facing the feed stream.

After passing through the last gasket 476, the draw solution and feedstreams exit the flow cell 400 through outlet ports 480, 482. In thisway, the flow cell 400 provides multiple water extraction processes ineach pass of the feed and draw solutions through the flow cell 400.

An alternate flow cell 500 is illustrated in FIG. 7. An envelope 506 ofmembranes layers 510 and supports 514 is inserted in a pressure vessel518. The envelope 506 is formed by rolling a flat membrane, thus formingthe membrane layers 510. The supports 514 are included between eachmembrane layer 510. The supports 514 provide support for the membranelayers 510, allowing fluid to flow on both sides of each membrane layer510.

The supports 514 may be of any suitable size, number, shape, anddimension, made of any suitable material, and placed in any suitablelocation. In one embodiment, the supports 514 are made of mesh,preferably plastic mesh. In another embodiment, the supports 514 areplastic rods. The supports 514 are preferably porous solids in order toprovide structural support to the membrane layers 510 while allowingfluid to flow between the supports 514. When draw solution solutionflows between the supports 514 and feed solution flows on the other sideof each membrane layer 510, water extraction can occur across eachmembrane layer 510.

The feed stream can be introduced into the envelope 506 by a centralinlet tube 530. From the central inlet tube 530, the feed solution isintroduced into the feed channels 534 between the membrane layers 510through holes (not shown) formed in the walls of the central inlet tube530, such as drilled holes. After passing through the channels 534, thefeed solution exits the envelope 506 though an outlet 540.

In a similar manner, the draw solution can be introduced into permeatechannels 544 containing the supports 514 though an inlet (not shown).After passing though the permeate channels 544, the draw solution exitsthe envelope 506 through an outlet 550.

Another alternate flow cell 560 is illustrated in FIGS. 8A and 8B. Oneor more cassettes 562 are covered on both faces 564 with a flat sheetmembrane 566 appropriate for membrane distillation or forward-osmosis,depending on the particular application. A porous support spacer 568 isplaced inside the cassette 562 between the two membranes 566. The spacer568 provides support for the membrane layers 566, allowing fluid to flowon both sides of each membrane layer 566 and allowing the cassette 562to be used under vacuum.

Each cassette 562 has one or more inlet ports 570 allowing draw solutionto flow into the cassette 562. Each cassette 562 also has one or moreoutlet ports 572 to allow diluted draw solution to flow out of thecassette 562. The flow inside the cassette 562 is under negativepressure (vacuum) when used for membrane distillation. The cassette (orcassettes) 562 is immersed in a tank 580 containing feed water. Althoughthe cassettes 562 and membranes 566 are shown as rectangular, othersizes and shapes of cassettes and membranes could be used, such assquare, round, or semi-circular cassettes and membranes. In addition,each cassette may have more than two faces 564. In particularimplementations, the membranes 566 are secured to each cassette 562,such as by an adhesive or a fastener, such as by tape, glue, clips,clasps, clamps, pins, or screws. Either feed or draw solution can flowinside the cassette.

The supports 568 may be of any suitable size, number, shape, anddimension and made of any suitable material. In one embodiment, thesupports 568 are made of mesh, such as a plastic mesh. In anotherembodiment, the supports 568 are plastic rods. The supports 568 arepreferably porous solids in order to provide structural support to themembrane layers 566 while allowing fluid to flow between the supports568. When permeate solution flows between the supports 568 and feedsolution flows on the other side of each membrane layer 566, waterextraction can occur across each membrane layer 566.

The feed stream can be introduced into the tank 580 by any suitableinlet, such as perforated inlet tubes 582 in the tank 580, such as inthe bottom of the tank 580. In a particular implementation, the inlet isconfigured to evenly distribute feed water in spaces between eachcassette 562. In further implementations the inlet is located elsewherein the tank 580 or is an inlet other than perforated inlet tubes 582.

From the perforated inlet tube 582, the feed solution is introduced intothe feed tank 580 between the membrane cassettes 562. After passingbetween the cassettes 562, the feed solution exits the tank 580 thoughan outlet 590.

The draw solution is introduced into the inside of the cassette 562through one or more inlet ports 570. After passing though the permeateporous support spacer 568, the diluted draw solution exits the cassette562 through the outlets 572. If desired, the permeate stream or feedstream can be passed through the flow cell 560 multiple times. In otherembodiments, the permeate stream or feed stream is removed from thesystem after one pass through the flow cell 560.

Examples of the First Exemplary Embodiment

A mathematical model was developed to predict the cost saving in a35,000 gallon-per-day desalination plant using forward-osmosis-assisteddesalination by reverse osmosis, as described in the foregoing exemplaryembodiments. Reverse osmosis-modeling software (ROSA®, Dow ChemicalCompany of Midland, Mich.) was used for modeling the reverse osmosisunit, and a self-developed modeling spreadsheet was used for modelingthe forward-osmosis stages. The reverse osmosis unit was modeled usingeight 8-inch Filmtec membranes (SW30-380, 35 m² membrane area permembrane element, available from Dow Chemical Company of Midland, Mich.)under operational parameters of 800 psi feed pressure and 45 gpm feedflowrate. The dual-stage forward-osmosis units were modeled as having atotal of 16 membrane elements each having an area of 35 m². Three caseswere modeled, including: (1) direct seawater desalination, (2) dilutedseawater desalination under similar operating conditions as with directseawater desalination, and (3) diluted seawater desalination at lowerfeed pressure than used for direct seawater desalination.

When comparing cases (1) and (2), the product-water recovery (ratiobetween product-water flowrate and feed flowrate to the desalinationunit) and water-production rate were both more than 30% higher thanobtained using conventional systems. When comparing cases (1) and (3),the model predicted that, under the same recovery and production rates(approximately 49% recovery) the reverse osmosis desalination unit couldbe operated at a feed pressure of 595 psi instead of 800 psi, with acorresponding increase in usable lifetime of the desalination unit.

EXAMPLE 1

This example sets forth the ROSA results obtained under case (1) notedabove.

System Summary:

Feed flow to stage 1 45.00 gpm Permeate flow 22.00 gpm Raw water flow tosystem 45.00 gpm Recovery 48.90% Feed pressure 800.00 psig Feedtemperature 20.00 C. Fouling factor 1.00 Feed TDS 34567.17 mg/L Chem.Dose none Number of elements 8 Total active area 3040.00 ft² Averagesystem flux 10.42 gfd Water classification seawater (open intake) SDI <5 Feed Feed Recirc Conc Conc Perm Avg Perm Boost Perm Flow Press FlowFlow Press Flow Flux Press Press TDS Stage Element #PV #Ele (gpm) (psig)(gpm) (gpm) (psig) (gpm) (gfd) (psig) (psig) (mg/L) 1 SW30-380 1 8 45.00795.00 0.00 23.00 765.56 22.00 10.42 0.00 800.00 451.98 (mg/L, exceptpH) Raw Water Adj Feed Permeate Concentrate NH₄ 0.00 0.00 0.00 0.00 K0.00 0.00 0.00 0.00 Na 10872.50 10872.51 168.60 21114.12 Mg 1298.661298.66 4.70 2536.74 Ca 413.76 413.76 1.47 808.24 Sr 0.10 0.10 0.00 0.20Ba 0.02 0.02 0.00 0.04 CO₃ 0.00 0.00 0.00 0.00 HCO₃ 0.00 0.00 0.00 0.00NO₃ 0.00 0.00 0.00 0.00 Cl 19271.49 19272.25 273.03 37450.90 F 12.9112.91 0.22 25.06 SO₄ 2694.04 2694.04 3.88 5268.01 Boron 0.00 0.00 0.000.00 SiO₂ 2.92 2.92 0.07 5.64 CO₂ 0.00 0.00 0.00 0.00 TDS 34592.3134567.17 451.98 67208.95 pH 7.60 7.60 7.60 7.60

Solubility Warnings:

BaSO₄ (% Saturation)>100%

CaF₂ (% Saturation)>100%

Scaling Calculations:

Raw Water Adj Feed Concentrate pH 7.60 7.60 7.60 Langelier SaturationIndex −5.79 −5.79 −5.23 Stiff & Davis Stability Index −6.75 −6.75 −6.42Ionic Strength (Molal) 0.72 0.72 1.44 TDS (mg/L) 34592.31 34592.3167208.95 HCO₃ 0.00 0.00 0.00 CO₂ 0.00 0.00 0.00 CO₃ 0.00 0.00 0.00 CaSO₄(% Saturation) 15.48 15.48 38.93 BaSO₄ (% Saturation) 70.68 70.68 161.15SrSO₄ (% Saturation) 0.19 0.19 0.50 CaF₂ (% Saturation) 9172.68 9172.6867471.10 SiO₂ (% Saturation) 2.54 2.54 4.91

To balance: 0.01 mg/L Na added to feed.

Array Details:

Perm Perm Feed Feed Feed Stage Ele- Flow TDS Flow TDS Press 1 mentRecov. (gpm) (mg/L) (gpm) (mg/L) (psig) 1 0.12 5.28 181.73 45.0034567.17 795.00 2 0.11 4.41 239.87 39.72 39140.54 789.30 3 0.10 3.58322.25 35.30 44002.44 784.49 4 0.09 2.81 439.87 31.73 48926.88 780.38 50.07 2.16 607.05 28.91 53644.99 776.80 6 0.06 1.62 842.11 26.76 57922.11773.62 7 0.05 1.24 1142.63 25.13 61611.17 770.73 8 0.04 0.90 1608.8523.89 64745.41 768.07

EXAMPLE 2

This example sets forth the ROSA results obtained under case (2) notedabove.

System Summary:

Feed flow to stage 1 45.00 gpm Permeate flow 28.81 gpm Raw water flow tosystem 45.00 gpm Recovery 64.01% Feed pressure 800.00 psig Feedtemperature 20.00 C. Fouling factor 1.00 Feed TDS 24535.12 mg/L Chem.Dose none Number of elements 8 Total active area 3040.00 ft² Averagesystem flux 13.64 gfd Water classification seawater (open intake) SDI <5 Feed Feed Recirc Conc Conc Perm Avg Perm Boost Perm Flow Press FlowFlow Press Flow Flux Press Press TDS Stage Element #PV #Ele (gpm) (psig)(gpm) (gpm) (psig) (gpm) (gfd) (psig) (psig) (mg/L) 1 SW30-380 1 8 45.00795.00 0.00 16.19 771.59 28.81 13.64 0.00 800.00 320.77 (mg/L, exceptpH) Raw Water Adj Feed Permeate Concentrate NH₄ 0.00 0.00 0.00 0.00 K274.50 274.50 4.78 754.27 Na 7546.29 7546.29 116.19 20762.55 Mg 901.36901.36 3.27 2498.84 Ca 287.48 287.48 1.02 797.01 Sr 5.69 5.69 0.02 15.77Ba 0.02 0.02 0.00 0.06 CO₃ 0.00 0.00 0.00 0.00 HCO₃ 0.00 0.00 0.00 0.00NO₃ 0.00 0.00 0.00 0.00 Cl 13604.71 13605.17 192.47 37462.95 F 9.12 9.120.18 25.00 SO₄ 1903.46 1903.46 2.78 5284.27 Boron 0.00 0.00 0.00 0.00SiO₂ 2.03 2.03 0.05 5.54 CO₂ 0.00 0.00 0.00 0.00 TDS 24552.33 24535.12320.77 67606.28 pH 7.60 7.60 7.60 7.60

Solubility Warnings:

BaSO₄ (% Saturation)>100%

CaF₂ (% Saturation)>100%

Scaling Calculations:

Raw Water Adj Feed Concentrate pH 7.60 7.60 7.60 Langelier SaturationIndex −5.94 −5.94 −5.10 Stiff & Davis Stability Index −6.79 −6.79 −6.29Ionic Strength (Molal) 0.50 0.50 1.44 TDS (mg/L) 24552.33 24552.3367606.28 HCO₃ 0.00 0.00 0.00 CO₂ 0.00 0.00 0.00 CO₃ 0.00 0.00 0.00 CaSO₄(% Saturation) 9.48 9.48 38.51 BaSO₄ (% Saturation) 64.44 64.44 216.68SrSO₄ (% Saturation) 10.01 10.01 40.20 CaF₂ (% Saturation) 3176.153176.15 66243.19 SiO₂ (% Saturation) 1.76 1.76 4.82

To balance: 0.00 mg/L Cl added to feed.

Array Details:

Perm Perm Feed Feed Feed Stage Ele- Flow TDS Flow TDS Press 1 mentRecov. (gpm) (mg/L) (gpm) (mg/L) (psig) 1 0.16 7.01 106.92 45.0024535.12 795.00 2 0.16 5.99 145.47 37.99 29044.26 789.51 3 0.15 4.90204.73 31.99 34457.96 785.17 4 0.14 3.81 297.61 27.09 40652.76 781.75 50.12 2.81 444.98 23.29 47253.31 779.01 6 0.10 1.99 678.46 20.47 53680.52776.75 7 0.07 1.36 1040.12 18.49 59371.06 774.83 8 0.05 0.93 1573.5517.13 64014.18 773.14

EXAMPLE 3

This example sets forth the ROSA results obtained under case (3) notedabove.

System Summary:

Feed flow to stage 1 45.00 gpm Permeate flow 22.05 gpm Raw water flow tosystem 45.00 gpm Recovery 49.01% Feed pressure 595.00 psig Feedtemperature 20.00 C. Fouling factor 1.00 Feed TDS 24535.12 mg/L Chem.Dose none Number of elements 8 Total active area 3040.00 ft² Averagesystem flux 10.45 gfd Water classification seawater (open intake) SDI <5 Feed Feed Recirc Conc Conc Perm Avg Perm Boost Perm Flow Press FlowFlow Press Flow Flux Press Press TDS Stage Element #PV #Ele (gpm) (psig)(gpm) (gpm) (psig) (gpm) (gfd) (psig) (psig) (mg/L) 1 SW30-380 1 8 45.00590.00 0.00 22.95 560.18 22.05 10.45 0.00 595.00 317.61 (mg/L, exceptpH) Raw Water Adj Feed Permeate Concentrate NH₄ 0.00 0.00 0.00 0.00 K274.50 274.50 4.78 533.75 Na 7546.29 7546.29 115.11 14689.09 Mg 901.36901.36 3.18 1764.69 Ca 287.48 287.48 1.00 562.84 Sr 5.69 5.69 0.02 11.14Ba 0.02 0.02 0.00 0.04 CO₃ 0.00 0.00 0.00 0.00 HCO₃ 0.00 0.00 0.00 0.00NO₃ 0.00 0.00 0.00 0.00 Cl 13604.71 13605.17 190.59 26499.18 F 9.12 9.120.18 17.71 SO₄ 1903.46 1903.46 2.69 3730.46 Boron 0.00 0.00 0.00 0.00SiO₂ 2.03 2.03 0.05 3.93 CO₂ 0.00 0.00 0.00 0.00 TDS 24552.33 24535.12317.61 47812.83 pH 7.60 7.60 7.60 7.60

Solubility Warnings:

BaSO₄ (% Saturation)>100%

CaF₂ (% Saturation)>100%

Scaling Calculations:

Raw Water Adj Feed Concentrate pH 7.60 7.60 7.60 Langelier SaturationIndex −5.94 −5.94 −5.39 Stiff & Davis Stability Index −6.79 −6.79 −6.46Ionic Strength (Molal) 0.50 0.50 1.00 TDS (mg/L) 24552.33 24552.3347812.83 HCO₃ 0.00 0.00 0.00 CO₂ 0.00 0.00 0.00 CO₃ 0.00 0.00 0.00 CaSO₄(% Saturation) 9.48 9.48 23.74 BaSO₄ (% Saturation) 64.44 64.44 138.86SrSO₄ (% Saturation) 10.01 10.01 23.43 CaF₂ (% Saturation) 3176.153176.15 23454.36 SiO₂ (% Saturation) 1.76 1.76 3.41

To balance: 0.00 mg/L Cl added to feed.

Array Details:

Perm Perm Feed Feed Feed Stage Ele- Flow TDS Flow TDS Press 1 mentRecov. (gpm) (mg/L) (gpm) (mg/L) (psig) 1 0.11 4.73 140.86 45.0024535.12 590.00 2 0.10 4.11 177.96 40.27 27399.74 584.31 3 0.10 3.49228.19 36.16 30493.95 579.42 4 0.09 2.90 296.65 32.67 33730.55 575.20 50.08 2.35 390.36 29.77 36989.11 571.52 6 0.07 1.87 518.40 27.41 40132.56568.26 7 0.06 1.46 692.42 25.54 43035.87 565.33 8 0.05 1.13 925.20 24.0845609.10 562.66

Forward-Osmosis Modeling:

Seawater-WW Effluent System

P_(OS) ^(SW) C_(SW) Q_(SW) Q_(WW) C_(WW) P_(OS) ^(WW) psi g/l l/minl/min g/l psi 1 331.05 27.84 67.48 16.19 0.44473 4.03467 20 2 330.0427.76 67.69 16.40 0.43915 3.98404 19 3 329.04 27.67 67.89 16.60 0.433723.93482 18 4 328.05 27.59 68.10 16.81 0.42844 3.88693 17 5 327.08 27.5168.30 17.01 0.42331 3.84033 16 6 326.10 27.42 68.50 17.21 0.418313.79496 15 7 325.14 27.34 68.71 17.42 0.41344 3.75077 14 8 324.19 27.2668.91 17.62 0.40869 3.70773 13 9 323.24 27.18 69.11 17.82 0.404073.66578 12 10 322.30 27.10 69.31 18.02 0.39956 3.62488 11 11 321.3827.03 69.51 18.22 0.39516 3.58500 10 12 320.45 26.95 69.71 18.42 0.390873.54609 9 13 319.54 26.87 69.91 18.62 0.38669 3.50812 8 14 318.63 26.8070.11 18.82 0.38260 3.47106 7 15 317.74 26.72 70.31 19.02 0.378613.43487 6 16 316.84 26.65 70.50 19.21 0.37472 3.39952 5 17 315.96 26.5770.70 19.41 0.37091 3.36499 4 18 315.08 26.50 70.90 19.61 0.367193.33124 3 19 314.21 26.42 71.09 19.80 0.36356 3.29826 2 20 313.35 26.3571.29 20.00 0.36000 3.26600 1 Module Recovery 19.05% Water Recovered3.81 L/min Flux 6.53 L/m²/hr

Seawater: ρ[psi]=CF*417.97

Effluent: ρ[psi]=CF*3.266

K membrane=3.5948*10⁻⁴ L/hr/psi

K 0.00035948 l/min/m²/psi ΔA 1.75 m² Total A 35 m² CS m² CS_(SWin)=27.84 g/l TDS C_(WWin)= 0.36 g/l TDS C_(CONCin)= 55.19 g/l TDS Q_(SWin)=67.48 l/min Q_(WWin)= 20 l/min Q_(CONCin)= 74.41 l/min

It is to be understood that the above discussion provides a detaileddescription of various embodiments. The above descriptions will enablethose skilled in the art to make many departures from the particularexamples described above to provide apparatus constructed in accordancewith the present invention. The embodiments are illustrative, and notintended to limit the scope of the present invention. Changes may bemade in the construction and operation of the various components,elements and assemblies described herein and changes may be made in thesteps or sequence of steps of the methods described herein. The scope ofthe present invention is rather to be determined by the scope of theclaims as issued and equivalents thereto.

1-17. (canceled)
 18. A water purification method comprising: passing astream of feed water and a stream of draw solution through a firstforward-osmosis unit to produce a diluted stream of draw solution;passing the diluted stream of draw solution through a desalination unitto produce a stream of product water and a stream of concentrated drawsolution; whereby desalinating the diluted stream of draw solutionrequires less energy than desalinating the draw solution.
 19. The methodof claim 18, further comprising passing the stream of concentrated drawsolution and a stream of salt water through a second forward osmosisunit to produce a diluted stream of concentrated draw solution.
 20. Themethod of claim 19, further comprising passing the diluted stream ofconcentrated draw solution through a third forward osmosis unit toproduce a feed stream for the desalination unit. 21-30. (canceled)